Generally speaking, an actuator is a mechanical device or system that is used to control another mechanical device or system. Most actuators take energy, typically in the form of a pressurized fluid (e.g., for pneumatic and hydraulic actuators) or electricity (e.g., for electric motors and solenoids) and convert it into mechanical motion that can be used to control or move the mechanical device or system.
While it is possible to design such actuators to have any of a wide range of operating characteristics and responses, it has proven considerably more difficult to design actuators that can faithfully reproduce or simulate the behavior of biological muscle contractions. For example, muscular motion in both humans and animals is dependent upon the storage of energy and the interaction of certain proteins upon stimulation by chemical entities such as calcium. The classical model of muscular movement is known as the “sliding filament theory.” However, this theory does not explain extremely rapid muscular movement such as a toad using the tongue to capture an insect, which is a near instantaneous muscle movement. Actuators that can more closely simulate or replicate biological muscle contractions could be used to advantage in a wide range of fields and applications, including the fields of robotics and prosthetics.